1. Field of the Invention
The invention relates to a micro-wave power amplifier, and more particularly to a micro-wave power amplifier which amplifies a micro-wave signal including a plurality of carrier frequencies.
2. Description of the Related Art
As an employing device constituting a micro-wave power amplifier used in a satellite or a base station in a mobile communication system, a GaAs field effect transistor is often used.
Such a micro-wave power amplifier is required to have high power and high efficiency performances in order to accomplish reduction in size and low power consumption. In addition, a micro-wave power amplifier is further required to have a function of concurrently amplifying a plurality of signals, since data to be transmitted or received increases. To this end, a micro-wave power amplifier is required to have low mutual modulation distortion and be superior in linearity, in order to avoid exerting harmful influences on other channels.
If a micro-wave power amplifier concurrently receives a plurality of signals, the micro-wave power amplifier would have secondary distortion power at a frequency equal to a difference frequency between the received signals as well as mutual modulation distortion, due to non-linearity of the micro-wave power amplifier.
A micro-wave power amplifier is generally designed to include either a plurality of field effect transistors arranged in parallel with one another in a multi-finger pattern, or a plurality of field effect transistor chips arranged in parallel with one another, to thereby widen a gate width for accomplishing high power output.
In such a high-powered micro-wave power amplifier, if a low-frequency impedance is increased to some degrees, secondary distortion power which is generated at a frequency equal to a difference frequency between the received signals would be increased, and would be mixed with an output signal at a drain of the field effect transistor, resulting in that the mutual modulation distortion would become worse than the distortion characteristic of the micro-wave power amplifier. Consequently, the linearity of the field effect transistor could not be effectively utilized.
A field effect transistor accomplishing high power output is generally designed to be of an internal matching type transistor where a plurality of field effect transistors operating in parallel with one another are matched with one another, from the standpoint of heat radiation and general use. When such a high-powered field effect transistor receives a micro-wave signal including a plurality of carrier frequencies, secondary distortion caused by a difference frequency among carrier frequencies would deteriorate the mutual modulation distortion.
In order to solve such a problem as mentioned above, Japanese Patent No. 3060981 (Japanese Unexamined Patent Publication No. 10-233638) has suggested a micro-wave power amplifier which can prevent deterioration in distortion characteristics, even if a micro-wave signal to be amplified includes a plurality of carrier frequencies.
FIG. 1 is a circuit diagram of the micro-wave power amplifier suggested in the above-identified Publication, and FIG. 2 is a block diagram of an example of the micro-wave power amplifier illustrated in FIG. 1.
With reference to FIG. 1, the micro-wave power amplifier is comprised of a signal input terminal 77 through which a signal is input into the micro-wave power amplifier, an input signal transmission path 71 through which a signal input through the signal input terminal 77 is transmitted to a package 70, a first capacitor 76 which is electrically connected between the signal input terminal 77 and the input signal transmission path 71 and removes dc current from the input signal, a gate bias applying terminal 75 through which a bias voltage is applied to a gate of a field effect transistor 61 arranged in the package 70, a first quarter wavelength path 73 electrically connected to the gate bias applying terminal 75, and transmitting a gate bias voltage applied through the gate bias applying terminal 75, to the field effect transistor 61, a gate protection resistor 72 electrically connected between the first quarter wavelength path 73 and the input signal transmission path 71, a first RF terminating capacitor 74 electrically connected at one end to the first quarter wavelength path 73 and grounded (86) at the other end, a signal output terminal 83 through which the package 70 transmits an output signal, an output signal transmission path 78 through which the package 70 transmits an output signal to the signal output terminal 83, a second capacitor 82 which is electrically connected between the output signal transmission path 78 and the signal output terminal 83 and removes dc current from the output signal, a drain bias applying terminal 81 through which a bias voltage is applied to a drain of the field effect transistor 61, a second quarter wavelength path 79 electrically connected to the drain bias applying terminal 81, and transmitting a drain bias voltage applied through the drain bias applying terminal 81, to the field effect transistor 61, and a second RF terminating capacitor 80 electrically connected at one end to the second quarter wavelength path 79 and grounded (87) at the other end.
The package 70 is comprised of the above-mentioned field effect transistor 61 having a grounded source, a gate electrode terminal 62 to which a gate of the field effect transistor 61 is electrically connected, a drain electrode terminal 63 to which a drain of the field effect transistor 61 is electrically connected, an input terminal lead 68 electrically connected between the input signal transmission path 71 and the gate electrode terminal 62, an input mating circuit 66 electrically connected between the input terminal lead 68 and the gate electrode terminal 62, an output terminal lead 69 electrically connected between the drain electrode terminal 63 and the output signal transmission path 78, an output matching circuit 67 electrically connected between the drain electrode terminal 65 and the output terminal lead 69, an difference frequency short-circuit inductor 65 electrically connected to the drain electrode terminal 63, and a difference frequency short-circuit capacitor 64 electrically connected at one end to the difference frequency short-circuit inductor 65, and grounded (85) at the other end.
The difference frequency short-circuit inductor 65 and the difference frequency short-circuit capacitor 64 define a difference frequency short-circuit circuit which is short-circuited at a difference frequency between carrier frequencies included in a micro-wave signal.
The input matching circuit 66, the first quarter wavelength path 73, the gate protection resistor 72 and the first RF terminating capacitor 74 define a gate bias circuit.
The output matching circuit 67, the second quarter wavelength path 79 and the second RF terminating capacitor 80 define a drain bias circuit.
FIG. 2 illustrates an example of a micro-wave power amplifier having such a circuit structure as illustrated in FIG. 1. The illustrated micro-wave power amplifier is of an internal matching type transistor where a plurality of field effect transistors operating in parallel with one another are matched with one another.
The micro-wave power amplifier is comprised of an input terminal 90 through which a micro-wave signal is received, a distribution circuit 91 which distributes the received micro-wave signal, matching circuit 92 ad 98 which match the received micro-wave signals with respect to an impedance by virtue of inductance and capacitance, field effect transistor chips 93 and 99 which amplify the distributed micro-wave signals, bonding patterns 97 and 101 arranged in the vicinity of drain electrodes of the field effect transistor chips 93 and 99, respectively, difference frequency short-circuit LC circuits each of which is comprised of a micro-strip path 102, 103 and a capacitor 104, 105, respectively, matching circuits 94 and 100 each of which matches a micro-wave signal having been amplified by the field effect transistor chips 93 and 99, with respect to an impedance by virtue of an inductance and/or a capacitance, a synthesizer circuit 95 which synthesizes the micro-wave signals with one another which signals have been matched by the matching circuits 94 and 100, and an output terminal 96 through which the micro-wave signals having been synthesized with one another are output.
Each of the micro-strip paths 102 and 103 is electrically connected to one end of the bonding pattern 97 and 101, respectively, and short-circuits distortion caused by a difference frequency between the carrier frequencies. Further, each of the micro-strip paths 102 and 103 has a length shorter than a quarter wavelength of the micro-wave signal.
When the micro-wave power amplifier having the above-mentioned structure receives a micro-wave signal having a plurality of carrier frequencies, there is generated distortion due to a difference frequency between the carrier frequencies. For instance, assuming that a micro-wave signal includes carrier frequencies f1 and f2, the distortion would have a frequency defined as an absolute value of a difference between the carrier frequencies f1 and f2, expressed as |f1xe2x88x92f2|. The difference frequency |f1xe2x88x92f2| increases secondary distortion, which results in an increase in mutual modulation distortion.
Hence, the micro-wave power amplifier illustrated in FIG. 2 was designed to decrease an impedance by resonating the difference frequency short-circuit LC circuits each comprised of the micro-strip path 102, 103 and the capacitor 104, 105, to thereby absorb the difference frequency distortion into a ground to smooth the difference frequency distortion for reducing the distortion caused by the difference frequency between the carrier frequencies.
However, the conventional micro-wave power amplifiers illustrated in FIGS. 1 and 2 are accompanied with problems as follows, if higher power is output from them.
In order to accomplish high power performance in the conventional micro-Wave power amplifier, the field effect transistor 61 is generally designed to have an increased gate width, as mentioned earlier. An increase in a gate width would cause an increase in output power transmitted from the field effect transistor 61, which further causes an increase in distortion caused by a difference frequency between carrier frequencies.
It was found out that the mutual modulation distortion was degraded as a difference between the input carrier frequencies became greater in the conventional micro-wave power amplifier, even if the difference frequency short-circuit circuit comprised of the difference frequency short-circuit capacitor 64 and the difference frequency short-circuit inductor 65 was electrically connected to the drain electrode terminal 63.
In order to make it possible to transmit data in a greater amount, a difference between carrier frequencies is required to be greater and a bandwidth is also required to be wider. However, the above-mentioned problems are bars to a greater difference in carrier frequencies.
FIG. 3 is a graph showing the tertiary mutual modulation distortion IM3 generated when a two-wave signal having frequencies f1 and f2 is input into the conventional micro-wave power amplifier in which the difference frequency short-circuit circuit is electrically connected to the drain electrode terminal 63 of the field effect transistor chip 61 having A total gate width of 600 mm.
Herein, the tertiary mutual modulation distortion IM3 is defined as follows.
IM3=2f1xe2x88x92f2
In FIG. 3, solid circles (xe2x97xaf) indicate tertiary mutual modulation distortion IM3 obtained when a two-wave signal having frequencies f1 of 2.1 GHz and f2 of 2.105 GHz is input into the micro-wave power amplifier, where a difference frequency is 5 MHz (0.005 GHz), and hollow circles (◯) indicate tertiary mutual modulation distortion IM3 obtained when a two wave signal having frequencies f1 of 2.1 GHz and f2 of 2.120 GHz is input into the micro-wave power amplifier, where a difference frequency is 20 MHz (0.02 GHz).
As is obvious in view of FIG. 3, IM3 (◯) obtained when the two-wave signal having a difference frequency of 20 MHz is input into the micro-wave power amplifier is degraded by about 5 dB in comparison with IM3 (xe2x97xaf) obtained when the two-wave signal having a difference frequency of 5 MHz is input into the micro-wave power amplifier.
As is understood in view of the explanation having been made above, in spite that a low frequency impedance is sufficiently reduced by electrically connecting the difference frequency short-circuit circuit to the drain electrode terminal 63 of the field effect transistor 61, the mutual modulation distortion is degraded as a difference between the input carrier frequencies becomes greater.
This is considered because regardless that a difference frequency corresponding to a difference between carrier frequencies caused by the non-linearity in the output side of the field effect transistor 61 is short-circuited by the difference frequency short-circuit circuit electrically connected to the drain electrode terminal 63, a difference frequency between carrier frequencies, caused by the non-linearity in the input side of the field effect transistor 61 which non-linearity is not ignorable due to an increase in a gate width, deteriorates the mutual modulation distortion.
Japanese Patent No. 2998837 (Japanese Unexamined Patent Publication No. 11-31923) has suggested a micro-wave frequency amplifier including a field effect transistor having a source grounded and carrying out non-linear operation in a micro-wave band, an input impedance matching circuit, and an output impedance matching circuit. The input impedance matching circuit is comprised of a fundamental wave input impedance matching circuit and a doubled-wave impedance control circuit which is electrically connected to a gate of the field effect transistor and which defines a resonance circuit which resonates at a frequency equal to a doubled frequency of a fundamental wave frequency. The output impedance matching circuit is comprised of a short-circuit device which is electrically connected to a drain of the field effect transistor and which short-circuits at the fundamental frequency, and a matching circuit which carries out impedance-matching to a doubled-wave of the fundamental wave.
However, the above-mentioned problems remain unsolved even in the above-identified Patent.
In view of the above-mentioned problems in the conventional micro-wave power amplifier, it is an object of the present invention to provide a micro-wave power amplifier in which distortion characteristic is not degraded, even though a difference between input carrier frequencies becomes greater.
There is provided a micro-wave power amplifier which amplifies a micro-wave signal including a plurality of carrier frequencies different from one another, including (a) a field effect transistor having a grounded source, (b) a first difference frequency circuit which is electrically connected to a drain of the field effect transistor, and is short-circuited at a difference frequency between the carrier frequencies, and (c) a second difference frequency circuit which is electrically connected to a gate of the field effect transistor, and is short-circuited at a difference frequency between the carrier frequencies.
For instance, the first difference frequency circuit may be comprised of (b1) an inductor which is electrically connected to a drain of the field effect transistor, and (b2) a capacitor which is electrically connected at one end to the inductor, and is grounded at the other end.
For instance, the second difference frequency circuit is comprised of (c1) an inductor which is electrically connected to a gate of the field effect transistor, and (c2) a capacitor which is electrically connected at one end to the inductor, and is grounded at the other end.
It is preferable that the first difference frequency circuit is comprised of a plurality of third difference frequency circuits, wherein each of the third difference frequency circuits is comprised of an inductor which is electrically connected to a drain of the field effect transistor, and a capacitor which is electrically connected at one end to the inductor, and is grounded at the other end, an inductance of the inductor and a capacity of the capacitor being determined such that the third difference frequency circuits are resonated with one another at the difference frequency.
It is preferable that the second difference frequency circuit is comprised of a plurality of third difference frequency circuits, wherein each of the third difference frequency circuits being comprised of an inductor which is electrically connected to a gate of the field effect transistor, and a capacitor which is electrically connected at one end to the inductor, and is grounded at the other end, an inductance of the inductor and a capacity of the capacitor being determined such that the third difference frequency circuits and resonated with one another at the difference frequency.
For instance, the inductor may be comprised of a bonding wire, and the capacitor may be comprised of a multi-layered ceramic capacitor.
The advantages obtained by the aforementioned present invention will be described hereinbelow.
In the above-mentioned conventional micro-wave power amplifier; if a field effect transistor is designed to have an increased gate width in order to accomplish high power output, difference frequency distortion, caused by a difference frequency between carrier signals which difference frequency is caused by the non-linearity in the input side of the field effect transistor, increases.
In contrast, in the micro-wave power amplifier in accordance with the present invention, the first difference frequency circuit comprise of an inductance and a capacitor is electrically connected to a drain of a field effect transistor, and further the second difference frequency circuit comprised of an inductance and a capacitor is electrically connected to a gate of the field effect transistor. As a result, a difference frequency between carrier frequencies, generated not only at the output side of the field effect transistor but also at the input side of the field effect transistor can be terminated by short-circuiting, ensuring removal of influence exerted on the mutual modulation distortion.
In addition, since the first and second difference frequency circuits are electrically connected to a gate and a drain of the field effect transistor, respectively, if a fundamental wave frequency band and a difference frequency band of a carrier frequency are sufficiently far away from each other, the difference frequency band can be terminated by short-circuiting without any influence to the fundamental wave frequency band. Hence, the micro-wave power amplifier in accordance with the present invention makes it possible to prevent degradation in the distortion characteristic, even if a difference between input carrier frequencies becomes greater.